This invention relates to a scanning type document imaging system and, more particularly, to a mechanism for adjusting the scanning speed to compensate for speed variations between scan and photoreceptor.
In the photocopier art, documents are reproduced onto a photoconductive imaging member at varying magnification ratios, the most common being a 1:1 magnification. At this 1X reproduction rate in a sanning type of optical system where a document in an object plane is incrementally illuminated and scanned and a flowing image projected upon a moving photoreceptor, the scanning speed must be exactly matched to the speed of the photoreceptor as it moves through the image plane. Any mismatch between the speeds results in a variation in the magnification ratio and/or a blurring of the output copy. As is known in the art, the scanning function can be enabled by movement of a document platen, or of the document alone, past a stationary exposure zone; alternatively, the document may be stationary and scanning optics, including a lamp/mirror combination, moved in a scan mode along the length of the document. Imaging systems are subject, with continued operation, to various changes in operating parameters which affect the original set of speed relationships. Typically, compensation is achieved by using variable speed drive systems which include servo motors to change the speed at which a document is scanned. A variable drive system encompasses a controller feedback circuit and a stepper or servo motor to adjust the relative speeds of the photoreceptor or the scanning mechanism either individually or in some combination. This type of prior art control system is relatively expensive because of the added cost of the controller and motors.
The following disclosures appear to be relevant:
U.S. Pat. No. 1,092,098 to Fitzgerald discloses a pulley whose diameter is varied by a rim movable supported over the diameter of a central hub.
U.S. Pat. No. 1,374,439 to Doherty discloses a device for increasing or decreasing the speed of a machine by shifting or changing the driving diameter of a belt pulley. A conical member is movable along a shaft 10 so as to change the working diameter of the pulley.
U.S. Pat. No. 2,552,179 to Kamp discloses a variable speed pulley which includes axially slideable mounted cones 5.
U.S. Pat. No. 3,604,281 discloses a variable speed device in which a belt support includes an elongated strip whose diameter is made variable.
U.S. Pat. No. 4,504,022 discloses an expandable feed roller comprising two discs, one of which is moveable axially along a shaft to change the effective diameter.
U.S. Pat. No. 4,487,518 to Enrini discloses an adjustable diameter pulley in which a plastic ring is elastically expandable about a tapered core assembly. The expansion is made variable by an engagement with a threaded nut which is located at the smaller end of the core assembly.
The present invention is directed to a more economical and simpler mechanism of adjusting speed ratios in a document scanning system. A mechanical control system is disclosed which is relatively inexpensive and which enables an operator to compensate for speed variations as they occur. The invention is more particularly directed towards a document imaging system, comprising, in combination,
means for moving a document across an exposure zone formed in an object scanning plane,
means for projecting a flowing light image of the document onto the surface of a photoreceptor moving at the same speed as the document,
an adjustable pulley speed mechanism connected to the moving photoreceptor and adapted to be driven at the same rotational speed,
a timing belt frictionally entrained between said pulley mechanism and said document moving means and
means for changing the effective diameter of said pulley mechanism so as to effect a change in the speed of the document movement across the exposure zone.